In an asynchronous digital radio transmission system in which autonomous distributed control is assumed, radio signals must be transmitted or received between radio stations where the radio signals can reach each station so as to control allocation of radio channels (radio frequencies).
In a system using a virtual carrier sense, communications are established between radio stations which perform transmission and reception of signals with each other by exchanging “a transmission request signal” and “a preparation completion signal”. Another radio station receiving the above signals postpones transmission of “a transmission request signal” based on “a transmission inhibition period” included in the received signal.
The virtual carrier sense has not been publicly known at the time of applying for a patent of the present invention as far as the applicant of the present invention knows.
In addition, the applicant could not find any technical report relating to the present invention. Therefore, technical documents relating to the present invention are not described herein.
However, existing technologies have the following problems.
Instead of direct communications between radio stations, when a multi-hop transmission is performed in which transmission communications between the radio stations are established by relay operations of another radio station located between the radio stations, transmission throughput is largely degraded.
FIG. 1 is a diagram showing a collision of transmission request signals in a multi-hop transmission. Referring to FIG. 1, a case is studied. In the case, when a radio station 13 and a radio station 14 establish communications by exchanging a transmission request signal and a preparation completion signal, a radio station 12 has transmission inhibition conditions. In this case, the transmission request signals from the radio station 13 to the radio stations 12 and 14 collide. As shown in FIG. 2, even if a radio station 11 transmits a transmission request signal to the radio station 12, since the radio station 12 is in the transmission inhibition conditions, the radio station 12 does not transmit a preparation completion signal. Consequently, the radio station 11 repeats transmitting the transmission request signal to the radio station 12. FIG. 2 is a diagram showing transmission inhibition conditions in the multi-hop transmission.
When the transmission inhibition conditions of the radio station 12 continue while the radio station 11 repeats transmitting the transmission request signal to the radio station 12, the radio station 11 discards data to be transmitted due to impossibility of communications. Consequently, the transmission throughput may be greatly degraded caused by discarding the data.
In addition, in a mesh network in compliance with IEEE 802.11, an exposed terminal problem may be generated due to a positional relationship among radio stations which desire to perform communications.
In this case, a packet is repeatedly transmitted based on operations stipulated in IEEE 802.11. However, even if repeated transmission of the packet is performed, when communications are not established, it is handled as packet loss. Consequently, communication quality is degraded. FIG. 3A is a diagram showing an exposed terminal problem. In FIG. 3A, a case is described in which communications between radio stations 11 and 12 and between radio stations 13 and 14 are performed using the same radio frequency (channel) f1. In a case where a signal from a radio station reaches only a nearest radio station, for example, during communications between the radio stations 13 and 14, communication quality between the radio stations 11 and 12 is greatly degraded.
When plural usable radio frequencies (channels) exist, the above quality problem can be solved by using a different radio frequency for communications between radio stations from a radio channel for communications between other radio stations. FIG. 3B is a diagram showing a solution to the exposed terminal problem. For example, as shown in FIG. 3B, a radio frequency f2 is used for communications between the radio stations 11 and 12, while the radio frequency f1 is used for communications between the radio stations 13 and 14.
However, when radio frequencies to use are arbitrarily determined between radio stations in the network, the number of radio frequencies becomes large beyond necessity.